If I understand this test, it does the following (Simon, please correct me if I'm wrong):
- set a timeout for 1s in the start callback to pause the animation using the CSS `animation` property shorthand halfway through
- set various timeouts for the animated value to be tested at 500ms (before the start callback timeout has elapsed and the animation is paused), at 1000ms and 2500ms to check the animation has been paused.
So the design of this test relies exclusively on setTimeout, which is bound to be flaky, especially as both the start callback and the second value check use the same timeout.
Let's see if we can respect the design of this test while making is robust.

I think it would be best to rewrite this test for the new animation engine to check the play state of the animation rather than be looking at CSS values. I'm going to do that, and leave the legacy variety of this test alone.